Three broadcasting systems are used for CATV broadcasting in the USA: namely, Standard Cable TV Signals (STD), Harmonic Related Carrier (HRC), and Incremental Related Carrier (IRC) systems.
FIG. 6 shows picture carrier frequencies (unit: MHz) of the individual channels (hereinafter “channel” may be abbreviated to “CH”) of the three CATV broadcasting systems. As shown in the figure, all the three CATV broadcasting systems, STD, HRC, and IRC, have different broadcasting frequencies in both CH5 and CH6. For broadcasting on CH5, for example, the STD system uses 79 MHz, HRC system uses 77.75 MHz, and IRC system uses 81 MHz. In this way, the broadcasting frequency varies with the broadcasting system.
Since different broadcasting systems use different broadcasting frequencies as described above, to receive signals of CATV broadcasting that has multiple receive modes, the channel tuning apparatus like conventional digital television signal receivers is configured to adopt the following procedures: when the presence or absence of signals on all channels are determined automatically, i.e., when automatic channel tuning is performed, a user must determine which broadcasting system is used on each channel for the CATV signals coming into the antenna terminal and then must set the receive mode that corresponds to the type of broadcasting system, on the channel tuning apparatus, before he/she can receive CATV broadcasts.
However, the conventional channel tuning apparatus described above has a problem of poor operability associated with the complex and troublesome manual operations during automatic channel tuning as described above.
Besides, with a CATV distribution system, in which the CATV broadcast signals sent from the central station are delivered to home users through a number of relay stations, it is necessary to take into consideration the fact that the signals may be frequency-shifted along the way due to some influence.
However, when the channel tuning apparatus such as a conventional digital television signal receiver is used for tuning, if the frequency of the desired channel is placed at the center as shown in FIG. 7, the frequencies within plus/minus 125 KHz from the frequency of the desired channel can be pulled in. This range is narrower by orders of magnitude than the pull-in range for typical analog signals, which is within plus/minus 1 MHz from the frequency of the desired channel.
Therefore, if the frequency of the signal delivered by a CATV broadcast exceeds the pull-in range of the desired channel selected by the user, there arises another problem that the conventional channel tuning apparatus cannot tune in the delivered signals.
As described above, the conventional channel tuning apparatus has poor operability during automatic channel tuning. Also, when selecting an unfamiliar CATV mode, the user may select a wrong mode, resulting in an inability to tune to a channel. Moreover, a proper channel cannot be tuned in if the frequency of the CATV broadcast signals is shifted from the frequency of the desired channel due to some influence.
The present invention has been made to solve the conventional problems described above. It provides a channel tuning apparatus which can improve operability by eliminating the need for a user to select an unfamiliar CATV mode during automatic channel tuning, allow reliable channel tuning without misjudging the presence or absence of signals even if the frequency of a CATV broadcast is shifted, and reduce the time required for automatic channel tuning.